Fifth wheels couplings are usually mounted on tractor trucks and in the engaged condition they receive the kingpin of a trailer. After the trailer is coupled on, the closing mechanism of the fifth wheel is moved from an open, ready to use position to a closed position by means of a handle situated on the fifth wheel or also by means of a remote controlled drive unit from the driver's cabin. For this, a closing hook belonging to the closing mechanism usually embraces at least partly the kingpin introduced into the fifth wheel. Depending on the specific structural design of the fifth wheel, the closing hook and the kingpin can additionally be secured by a closing bar. When driving on the road, the closing hook in particular, with its contact surfaces at the kingpin and the closing bar, but also the closing bar with its contact surface at the closing hook, is subjected to extreme stresses in many respects.
The stresses are to a large extent produced by a relative movement between the kingpin and the frictional surfaces of the closing hook or the closing bar which make contact with it, in additional to further impact stresses, so that even after a short time both the kingpin and the closing hook will have their material worn away. The relative movement results from the steering movements of the tractor truck. This mechanical loading is further aggravated by corrosive influences, for example, those produced by contact with diesel fuel and unavoidable sand getting into the region of the closing mechanism.
The overwhelming majority of closing hooks are furnished with a coating by the manufacturer. This coating is a corrosion protection layer produced by a cathodic paint-dip method.
In order to minimize wear and tear, the problematical region of the frictional surfaces on the closing hook and/or closing bar is intensively lubricated with grease. This can be done manually by the driver, for example, or also automated via the central lubricating system of the tractor truck, if present. One such lubricating device is mentioned, for example, in DE 94 01 718 U1. One major component of this is a distribution block, arranged on the bottom side of the coupling plate, which is connected by a supply line to a central lubricant reservoir and from which a lubricating line runs to the closing hook, among other things. The lubricating of the closing hook occurs from the lubricant reservoir via the supply line through the distribution block and the lubricating line.
The major drawback of this lubrication system is the high grease consumption of several kilograms per year and the accompanying environmental pollution. Furthermore, solids build up in the excess grease, and during drive operation they are drawn in by a pump effect between kingpin and the frictional surfaces, where they bring about further intensification of the wear process.
For this reason, efforts have already been made to lower the grease consumption by coatings on the closing hook or kingpin. One such fifth wheel is described in DE 196 24 803 A1. The antifriction layer used there consists of a carbon-based layer, by which is meant pure carbon layers of diamondlike structure, metal-containing carbon layers or amorphous carbon layers, either pure or metal-containing. The major drawback of the known coating is its extremely great hardness as compared to the much more elastic material of the closing hook. In practice, this leads to an egg shell effect, that is, a two-dimensional chipping of the coating and a resulting high coefficient of friction, as well as a concomitant rapid destruction of the closing hook.
A different coating of the closing hook and the closing bar of a fifth wheel is disclosed by DE 43 04 857. The coating comprises a layer of hard material, sprayed onto the base material, and on top of that a sliding layer, which sliding layer contains a PTFE material. Furthermore, the thickness of the sliding layer is less than 50 μm and is dimensioned so that the sliding layer thinly covers the peaks of the layer of hard material and fills up the valleys lying between them.
However, a completely grease-free system has not worked out well in practice, since the wear on the components involved is too great under a constant loading.
Therefore, the basic problem of the invention is to improve a system for lubricating the closing mechanism and the individual components of which it is made so that the grease consumption under high operational availability is significantly and enduringly reduced, whether or not a central lubricating system is present.